In light waveguide communication systems, high exploitation of the transmission capacity of the light waveguide can be achieved in a bidirectional (or unidirectional) wavelength-division multiplex (WDM) in that given a bidirectional wavelength-division multiplex, the light waves having a lower wavelength, for instance, around 830 nm, are employed for the signal transmission in one transmission direction and light waves having a higher wavelength, for instance, in the range of 1300 nm, are utilized for the signal transmission in the opposite or other direction. Appropriate optical separating modules are utilized at both ends of the light waveguide link for separating the two wavelengths and these separating modules are respectively coupled to transmission modules containing electro-optical transducers and to a reception module containing an optoelectrical transducer.
An optical separating module (WDM coupler) which can be a wavelength division multiplexer or demultiplexer can be comprised of a wavelength-selective interference filter or beam splitter layer which is composed of a plurality of layers and which is vapor-deposited onto an oblique end face of an optical fiber in a high vacuum. This beam splitter layer, which lies on a slanting plane, separates two end faces of two mutually aligned optical fibers. An additional optical fiber which will extend obliquely relative to the aligned two optical fibers and has its axis intersecting the common axis of the two waveguides in the plane of the beam-splitting layer is provided to receive light reflected by the beam-splitting layer. Such a device is provided by placing optical fibers in V-shaped grooves of optical or glass members which are joined together, cut, polished and have the beam-splitting layer applied and then are subsequently joined and then separated again to form the various units. Examples of these are disclosed in U.S. patent application Ser. No. 246,100, filed Mar. 20, 1981, which issued as U.S. Pat. No. 4,637,682 on Jan. 20, 1987 and is based on German OS No. 3,012,184. Also, these are disclosed in an article in Applied Optics, Vol. 20, No. 18, 1981, pp. 3128-3135.
In order to boost the optical cross-talk attenuation of the separating modules or WDM couplers, discrete band-pass filter laminae are provided at the reception optical fibers (see article from Applied Optics). Filter laminae can also be provided in a corresponding way in a bidirectional light waveguide transmission system which does not have separating modules as disclosed in U.S. Pat. No. 4,300,811, whose disclosure is incorporated by reference.
In a bidirectional light waveguide transmission system comprising two optical separating modules of the above-mentioned type, the transmission module as well as the reception module at the two ends of the light waveguide link can (as indicated in IEEE Journal of Light Waveguide Technology, LT-2 l (1984), October, No. 5, pp. 675-681, particularly in FIG. 1, and as also indicated in French Patent No. 2,509,479, particularly FIG. 2 can be respectively coupled to the same optical fiber of the particular separating modules of the two separating modules. The two separating modules are provided with beam splitters having mutually offset filter characteristics in accordance with the two light wavelengths utilized for the two transmission directions. This enables uniform structure transmit-receive modules to be employed at both ends of the optical transmission link with each module comprising a transmission module, reception module and separating module. A uniform assembly adjustment and other test devices can accordingly be employed in the fabrication of each of these modules.
The optical cross-talk attenuation of, for example, 20 dB achieved with the beam-spiltter layer can be boosted with the assistance of a discrete band-pass filter laminae which is situated in front of the reception module. The insertion of such a filter laminae, however, will entail an insertion attenuation of about 1 dB or more.